15441 Computer Networking

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15-441 Computer Networking

• Introduction, Part II

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Introduction, Part II

• Overview
• whats the Internet
• whats a protocol?
• network edge
• network core
• access net, physical media
• performance loss, delay
• protocol layers, service models
• backbones, NAPs, ISPs
• history
• ATM network

• Chapter goal
• get context, overview, feel of networking
• more depth, detail later in course
• approach
• descriptive
• use Internet as example

today
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Oops!
I said something outrageously wrong last time!!
Looks like stocks will keep rising indefinitely!
What was it?
I said that TCP doesnt provide a data integrity
check. It does.
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Delay in packet-switched networks

• nodal processing
• check bit errors
• determine output link
• queueing
• time waiting at output link for transmission
• depends on congestion level of router

• packets experience delay on end-to-end path
• four sources of delay at each hop

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Delay in packet-switched networks

• Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

Note s and R are very different quantities!
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Queueing delay (revisited)

• Rlink bandwidth (bps)
• Lpacket length (bits)
• aaverage packet arrival rate

traffic intensity La/R

• La/R 0 average queueing delay small
• La/R -gt 1 delays become large
• La/R gt 1 more work arriving than can be
  serviced, average delay infinite!

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Protocol Layers

• Networks are complex!
• many pieces
• hosts
• routers
• links of various media
• applications
• protocols
• hardware, software

• Question
• Is there any hope of organizing the structure of
  a network?
• Or at least our discussion of networks?

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Organization of air travel

• a series of steps

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Organization of air travel a different view
baggage check runway takeoff airplane
routing
baggage claim runway landing airplane
routing
airplane routing

• Layers each layer implements a service or
  services
• via its own internal-layer actions
• relying on services provided by layer below

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Layered air travel services
check-in-counter-to-baggage-claim
delivery people transfer loading gate to
arrival gate runway-to-runway delivery of plane
bag transfer belt at check-in counter to belt at
baggage claim
airplane routing from source to destination
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Distributed implementation of layer functionality
baggage (check) gates/bags (load) runway
takeoff airplane routing
baggage (claim) gates/bags (unload) runway
landing airplane routing
arriving airport
departing airport
intermediate air traffic sites
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Why layering?

• Dealing with complex systems
• explicit structure allows identification,
  relationship of complex systems pieces
• layered reference model for discussion
• modularization eases maintenance, updating of
  system
• change of implementation of layers service
  transparent to rest of system
• e.g., change in gate procedure doesnt affect
  rest of system
• layering considered harmful?

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Internet protocol stack

• application supporting network applications
• ftp, smtp, http
• transport host-host data transfer
• tcp, udp
• network routing of datagrams from source to
  destination
• ip, routing protocols
• link data transfer between neighboring network
  elements
• ppp, ethernet
• physical bits on the wire

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Layering logical communication

• Each layer
• distributed
• entities implement layer functions at each node
• entities perform actions, exchange messages with
  peers

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Layering logical communication

• E.g. transport
• take data from app
• add addressing, reliability check info to form
  datagram
• send datagram to peer
• wait for peer to ack receipt
• analogy post office

transport
transport
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Layering physical communication
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Protocol layering and data

• Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below

source
destination
message
segment
datagram
frame
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Internet structure network of networks

• roughly hierarchical
• national/international backbone providers (NBPs)
• e.g. BBN/GTE, Sprint, ATT, IBM, UUNet
• interconnect (peer) with each other privately, or
  at public Network Access Point (NAPs)
• regional ISPs
• connect into NBPs
• local ISP, company
• connect into regional ISPs

regional ISP
NBP B
NBP A
regional ISP
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National Backbone Provider
e.g. BBN/GTE US backbone network
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Internet History
1961-1972 Early packet-switching principles

• 1961 Kleinrock - queueing theory shows
  effectiveness of packet-switching
• 1964 Baran - packet-switching in military nets
• 1967 ARPAnet conceived by Advanced Reearch
  Projects Agency
• 1969 first ARPAnet node operational

• 1972
• ARPAnet demonstrated publicly
• NCP (Network Control Protocol) first host-host
  protocol
• first e-mail program
• ARPAnet has 15 nodes

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Internet History
1972-1980 Internetworking, new and proprietary
nets

• 1970 ALOHAnet satellite network in Hawaii
• 1973 Metcalfes PhD thesis proposes Ethernet
• 1974 Cerf and Kahn - architecture for
  interconnecting networks
• late70s proprietary architectures DECnet, SNA,
  XNA
• late 70s switching fixed length packets (ATM
  precursor)
• 1979 ARPAnet has 200 nodes

• Cerf and Kahns internetworking principles
• minimalism, autonomy - no internal changes
  required to interconnect networks
• best effort service model
• stateless routers
• decentralized control
• define todays Internet architecture

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Internet History
1980-1990 new protocols, a proliferation of
networks

• 1983 deployment of TCP/IP
• 1982 smtp e-mail protocol defined
• 1983 DNS defined for name-to-IP-address
  translation
• 1985 ftp protocol defined
• 1988 TCP congestion control

• new national networks Csnet, BITnet, NSFnet,
  Minitel
• 100,000 hosts connected to confederation of
  networks

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Internet History
1990s commercialization, the WWW

• Early 1990s ARPAnet decomissioned
• 1991 NSF lifts restrictions on commercial use of
  NSFnet (decommissioned, 1995)
• early 1990s WWW
• hypertext Bush 1945, Nelson 1960s
• HTML, http Berners-Lee
• 1994 Mosaic, later Netscape
• late 1990s commercialization of the WWW

• Late 1990s
• est. 50 million computers on Internet
• est. 100 million users
• backbone links running at 1 Gbps

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ATM Asynchronous Transfer Mode nets

• Internet
• todays de facto standard for global data
  networking
• 1980s
• telcos develop ATM competing network standard
  for carrying high-speed voice/data
• standards bodies
• ATM Forum
• ITU

• ATM principles
• small (48 byte payload, 5 byte header) fixed
  length cells (like packets)
• fast switching
• small size good for voice
• virtual-circuit network switches maintain state
  for each call
• well-defined interface between network and
  user (think of telephone company)

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ATM layers

• ATM Adaptation Layer (AAL) interface to upper
  layers
• end-system
• segmentation/reassembly
• ATM Layer cell switching
• Physical

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Chapter 1 Summary

• Covered a ton of material!
• Internet overview
• whats a protocol?
• network edge, core, access network
• performance loss, delay
• layering and service models
• backbones, NAPs, ISPs
• history
• ATM network

• You now hopefully have
• context, overview, feel of networking
• more depth, detail later in course